1. Field of the Invention
The present invention relates to an electronic circuit board capable of highly densely mounting LSI chips such as CPU, memory, and the like. More specifically, the present invention relates to an opto-electronic circuit board (or an optical integrated circuit board) having an electrical wiring layer and an optical wiring layer.
2. Related Background Art
A cellular phone and a portable digital assistant (PDA) need to provide high-speed processing and compactness or light weight at the same time. However, it is pointed out that accelerating the processing speed increases the influence of wiring delay in the electronic circuit board. To minimize this influence, the simplest method is to make the wiring in a chip and between chips as short as possible. Since it can also miniaturize circuit boards, many inventions have been made for this purpose.
As the processing speed increases, however, another problem appears, i.e., a noise due to electromagnetic interferences (EMI).
Since electronic parts are arranged very closely to each other, the wiring becomes shorter but the wiring density becomes higher. When a high-speed signal is applied to adjacent signal lines, electromagnetic waves interfere with each other due to the mutual electromagnetic induction, causing a noise. As a result, the signal cannot be transmitted correctly. Mobile terminals, in particular, are more and more designed for low voltages, and are consequently driven with a large current, increasing effects of EMI.
This phenomenon means that the mobile terminal is susceptive to the external radio environment, i.e. so-called immunity or electromagnetic compatibility (EMC). That is, the fact that the mobile terminal itself easily generates EMI means that it easily sense an external electromagnetic field. Accordingly, normal data processing is unavailable depending on the radio environment.
Usually, these problems are solved by multilayering a ceramic substrate and enhancing EMC (i.e. electromagnetic compatibility) for each layer. However, it raises problems about costs and yield ratios, while it is essentially impossible to provide an EMI-free environment.
On the other hand, there is proposed a method of using optical wiring that essentially has an advantage of no electromagnetic induction.
According to Japanese Patent Application Laid-Open No. 9-96746 (“Active optical circuit sheet or active optical circuit board” by Yoshimura et al.), for example, an optical wiring section is separated from an electrical wiring section. Based on a signal voltage from an electronic instrument, an optical switch or an optical modulator converts an electric signal into an optical signal for transmission. The optical signal is reconverted into an electric signal by a light receiving element that is provided at a different position in the optical wiring section. Thus, an electric connection is made with another or the same instrument.
This method excels in compensating the demerit of the electrical wiring for the optical wiring. Since the optical wiring uses a one-dimensional optical waveguide (or fiber), however, it is necessary to predetermine a position for optical wiring. Further, the size for the optical wiring becomes much larger than that for the equivalent electrical wiring.
According to Japanese Patent Application Laid-Open No. 11-196069 (“Optical signal transmission apparatus, optical signal transmission method, and signal processing apparatus” by Sakai et al.), the signal light input/output section is arranged at each of both opposite ends of the two-dimensionally extending optical sheet bus. An optical signal is two-dimensionally transmitted in the optical sheet bus from one end. The light receiving element at the other end converts the optical signal into an electric signal. This method solves conventionally inevitable delays and transmission speed limitations in the electrical wiring and provides easy mounting. There are arranged a transmission device (e.g., one-dimensional semiconductor laser array) and a reception device (e.g., one-dimensional photodiode array) at both ends of a two-dimensional optical waveguide called an optical sheet (having a waveguide structure only in the thickness direction).
The signal operation is summarized as follows. A logic signal from the electric circuit directly drives the semiconductor laser to convert the electric signal into the optical signal. The generated light propagates inside the optical sheet. At this time, the light is guided in the thickness direction and freely propagates in the plane direction perpendicular thereto. As a result, through the necessary optical power becomes larger than the conventional one, to increase the load to the electrical circuit, tolerances for mounting the optical sheet and optical devices become much larger. This method also solves conventionally inevitable delays and transmission speed limitations in the electrical wiring.